The present invention relates to a control circuit for proportional electro-hydraulic fluid control valves.
In the past, proportional electro-hydraulic fluid control valves have been generally so designed that the flow rate or pressure of a fluid is controlled in the form of a so-called open loop control by the self-adjusting function incorporated in the valve in accordance with a present input signal to its electric control system. In this case, there is the disadvantage of the flow rate or pressure being changed by variations in the external conditions such as the temperature of the fluid. Thus, attempts have been made to eliminate this disadvantage as shown in U.S. Pat. No. 4,285,639, for example, by detecting the flow rate or pressure as an electric signal by a sensor and feeding back to the electric control system, thereby performing a closed loop control to improve the static accuracy against variations in the external conditions.
Where this closed loop control is used, however, there is the danger of deteriorating the performance than in the case of the open loop control depending on the actual use conditions of the electric system and the hydraulic fluid system of the valve.